Pilot specification for OL rank-1 region Document Number: IEEE C80216m-09_1915 Date Submitted: Source: Kiran Kuchi, J. Klutto Milleth, Padmanabhan M SVoice: CEWiT, India Venue: Comments on P802.16m/D1 Base Contribution: None Purpose: To discuss and adopt the proposed text in section To discuss in TGm for appropriate action. Notice: This document does not represent the agreed views of the IEEE Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and. Further information is located at and.

Background For a reuse 1 system, the strongest interference for a cell edge user comes from those sectors with sector numbers different from its desired one The SINR seen by the pilot symbols can be significantly improved by avoiding pilot to pilot collisions between sectors  Improves Channel Estimation  Improves Covariance Estimation

Interlaced Pilots Interlaced pilots use pilot patterns which are cyclically shifted, resulting in the collisions between pilot and data. But, the use of interlaced pilots (with power boosting) has the following disadvantages :  It reduces the data SINR.  Interference suppression receivers can not function smoothly, because of the mismatch between the estimated covariance and the actual covariance in the data locations that do not have power boosting  With CDR encoding, conjugate data pairs collide with pilots. Therefore, the number of samples available for interference covariance estimation is half – compared to pilot-on-pilot case.  In general, poor CDR performance with interlaced pilots

Pilot on Pilot An option for CDR is to use 2-stream pilots in pilot-on-pilot mode: Total 12 pilot tones Good quality interference covariance estimation requires very high quality channel estimates The pilot sequence used by different base stations determine the quality of channel and interference covariance estimates Poor CDR performance with PRBS pilots in pilot-on-pilot mode. Need to use pilot sequencers with low-cross correlation properties. Re-use 1 needs 21 pilot sequences with low-cross correlation  12-pilots tones not enough to design 21 pilot codes The number of active interferers may vary from RB to RB. Pilots from neighboring RBS cannot be used improve interference covariance estimates

Collision Free Interlaced Pilots(CoFIP) Interference to the pilot tones from the data tones of the other sectors is completely avoided. A sector transmits null tones at the pilot locations of the other sectors. The accuracy of channel/interference covariance estimation improves because:  the pilot tones are almost interference free (only weak higher tier interference)  The signal received on the null tones can be used to estimate the covariance or to explicitly Also, possible to estimate two dominant interferers Implementation Complexity  Reliable channel estimation and interference covariance estimation with single RB processing  Channel estimation complexity same as that of conventional 2D-MMSE  Interference covariance estimation using null tones and pilot tones Very high CDR performance

Sector 0 0P 00P 0 P00P00 00P0 0 P P denotes pilot tone and 0 denotes null tone P0 0 P P 00 P 0P00 P0 Previously proposed CoFIP Pattern 00 P 00 P 0P00P0 P00P 00 Sector 1Sector 2 CDR requires subcarriers in pair, which is not possible with this pilot structure Needs even number of data subcarriers per OFDM symbol

Sector 0 0P 00P 0 0P0 P00 00P0 0P P denotes pilot tone and 0 denotes null tone P0 0 P0 0 00P 0P0 0P00 P0 The new CoFIP Pattern for irregular sub frames with 6 OFDM symbols 00 P 00 P P00 0 0P P00P 00 Sector 1Sector 2 Even number of data subcarriers in every OFDM symbols in each PRU Enables simple subcarrier mapping for CDR with all advantages of CoFIP

Sector 0 0P 00P 0P00 P0P0 00P0 0 P denotes pilot tone and 0 denotes null tone P0 0 P0 000P 0P00 0P00 P The new CoFIP Pattern for irregular sub frames with 5 OFDM symbols 00 P 00 P0P0 0 00P P00P 0 Sector 1Sector 2 For 18 X 7 RBs, add the first OFDM OFDM symbol after 6 th OFDM

Estimation of interference covariance The interference covariance matrix is estimated using all the symbols received at the pilot/null locations The covariance is estimated from the pilot tones corresponding to the same sector number, and from the null tones of different sector numbers The CQI estimates are very stable Channel estimation and interference covariance estimation confined to 1 RB processing

Pilot sequences for CoFIP The pilot sequences assigned to different cells should have low cross correlation within the space of an RB. Some of the possibilities are:  Use of short pilot sequences, which have low cross-correlation within a localized resource unit.  Use of long PN sequences Results in increased cross correlation

Short PN sequence Seven PN sequences are generated using an LFSR with the characteristic polynomial X 3 +X +1 The first 6 samples of a sequence is used for deriving the 6 pilots with in a PRU Two pilot allocation methods are suggested:  Method 1: The same sequence is used in different PRUs of a cell  Method 2: The sequence used in different PRUs of a cell are different Definitions:  A cell is a collection of 3 sectors and is assigned unique id = CellID  PRU index ‘s’ in frequency and sub-frame number ‘t’ in time

Method 1 The pilot code used in a cell is C( CellID%7 ), where C(i) is the i th code of the set of codes C, where i=0,1,..6  Some minimal pilot planning is required c6c6 c1c1 c0c0 c5c5 c4c4 c3c3 c2c2 Note that the sectorization in the hexagon cell is not shown due to the fact that same PN sequence is used in all sectors in a cell

Method 2 Pilot sequence cycling in time and/or frequency is implemented to exploit the advantage of interference averaging on the pilots. The pilot code used in a cell with CellID = k is C( (s+t+k)%7), where C(i) is the i th code of the set of codes C. i=0,1,…6 cell_ID = 0 cell_ID=1 cell_ID=2 C0C0 C1C1 C2C2 C3C3 C4C4 C1C1 C2C2 C3C3 C4C4 C5C5 C2C2 C3C3 C4C4 C5C5 C6C6 C3C3 C4C4 C5C5 C6C6 C0C0 C1C1 C2C2 C3C3 C4C4 C5C5 C2C2 C3C3 C4C4 C5C5 C6C6 C3C3 C4C4 C5C5 C6C6 C0C0 C4C4 C5C5 C6C6 C0C0 C1C1 C2C2 C3C3 C4C4 C5C5 C6C6 C3C3 C4C4 C5C5 C6C6 C0C0 C4C4 C5C5 C6C6 C0C0 C1C1 C5C5 C6C6 C0C0 C1C1 C2C2 s t -->

Long PN sequence Long PN sequences are generated using an LFSR with the characteristic polynomial X 11 +X 9 +1 The LFSR is initialized as follows:  b 0...b 6 = (CellID % 128 ) + 1  b 7...b 10 = (symbol index % 16) For the i th OFDM symbol, a PN sequence PN i (k), k=0,1,2,....(FFTSI-1) is generated The pilot tone (i,k) ( i th OFDM symbol, k th sub-carrier ) is modulated with PN i (k) Pilot planning is not necessary

Comparison of Various Pilot Modes Pilot-on-DataPilot-on-PilotCOFiP Total overhead5.5%11.1%16.6% Channel estimation complexity LowMediumLow PerformanceVery PoorPoorHigh 1) Pilot-on-Data is not suitable for CDR 2) COFiP needs 5.5% more overhead compared to COFiP 3) In spite of additional overhead, COFiP outperforms pilot-on-pilot significantly

Conjugate Data Repetition (CDR) with CoFIP Simulation parameters  2Tx antenna – 2Rx antenna  Channel: Modified Ped B, 7 Hz doppler  3 interferers of equal power. One interferer colliding with pilot tones and the remaining two interferers on null tones (Some what worst case scenario. In practice, the CCI on pilots is much weaker)  Thermal noise level is 10 dB below the power of one of the interferers.  Twenty 18x6 RBs, 4 in freq. and 5 in time. All estimation is done within a single RB  2D-MMSE did not exploit knowledge of pilot sequence used by the CCI which collides with pilot tones  The new CoFIP pilot pattern is used.  Performance with Short and long PN pilot sequences are compared.

CDR performance with different short PRBS

CDR Performance comparison between short and log PRBS

Conclusions COFiP  Low implementation complexity  Additional overhead gives robustness in design, reduces implementation complexity, higher overall system performance  Recommends COFiP with short PRBS 7- pilot codes with low-cross correlation with minimal pilot planning Pilot-on-Pilot with long PRBS significantly worse than COFiP

Proposed Text OL rank-1 uses collision free interlaced pilots. The index of the COFiP PRU type used by a particular BS with Cell_ID=k is denoted by pk. The index of the used CoFIP type PRU is determined by the Cell_ID according to the following equation: p k = mod(k,3) In the Fig xxxx ‘X’ donates a null tone i.e., no data or pilot tone is transmitted in that location COFiP Type 0 PRU COFiP Type 1 PRUCOFiP Type 2 PRU 0P 00P 0 0P0 P00 00P0 0P P0 0 P0 0 00P 0P0 0P00 P0 00 P 00 P P00 0 0P P00P 00

Proposed Text Pilot Sequence Definition:  Define 7- codes: C(j), j=0..6. [TBD]  The pilot code used in a cell with Cell_ID = k is P=C( (s+t+k)%7), where C(i) is the i th code of the set of codes C(i). i=0,1,…6  P=[p(0) p(1) … p(5)] where p(i), i=0,..,5 is the ith pilot symbol in Fig xxx